The indicated circuit uses an unusual buffer from National Semiconductor. The frequency is given by resistors R1 and R2 as well as capacitors C1 and C2.
This shield, found in old English documentation, appears with variations elsewhere in this section...
This complementary or bilateral emitter follower was found in American documentation from the 90s....
This configuration that functions as a zener diode was found in an English publication from 1976....